In conventional DC power supply systems for supplying electrical power directly to communication devices, such as those used for telephone switching, backup storage batteries are maintained by the floating charge method. In such systems, a load and a storage battery are connected in parallel to an output of a rectifier so that the rectifier continuously supplies DC power to the load and a required charging current to the storage battery to fully charge it according to the charged state of the battery. In this system, if a power failure or a rectifier malfunction occurs, the storage battery immediately starts discharging to avoid interruption of power supplied to the load even for a moment.
Since a storage battery is employed in such systems where high reliability is required, the state of the system should desirably be determined by examining the remaining capacity of the storage battery to see whether the storage battery can achieve a sufficient performance.
In view of these circumstances, some methods for checking and monitoring the state of a backup storage battery have been studied. In one of these methods, it is checked whether the power supply function of a battery works well under a normal operation condition where a power supply system is supplying electrical power to an actual load. In this method, the storage battery is discharged by reducing the output voltage of the rectifier for a predetermined time to examine the voltage characteristic during the discharge period (refer to KOZUKA, et al. “Development of On-line Battery Testing Technology”, Proceedings of INTELE '97, 18-2, P. 397).
However, the above-described conventional technology, where the output voltage of the rectifier is reduced only for a predetermined time to carry out discharge the storage battery, is intended to check whether the charging and discharging circuits of the storage battery are operating normally. Therefore, the technology is not appropriate for obtaining the remaining capacity of the storage battery.
Even if an attempt is made to estimate the remaining capacity of the storage battery, constant discharging current from the storage battery will not be guaranteed since the discharging current flows directly through an actual fluctuating load. For this reason, the discharge-voltage characteristic obtained in this manner differs from the constant-current discharge characteristic and hence cannot be used to estimate the remaining capacity of the storage capacity.
Furthermore, the fact that the relationship between the remaining capacity and the voltage characteristic of the storage battery has not been clarified is the most significant bottleneck to estimating the remaining battery capacity.